Add the clangd cache to the gitignore

[madd-games]

clangd reads the /compile_commands.json file and provides IDEs such as VSCode with features like "go to definition", "find all references", hints, etc. It creates a directory called .cache in the package root, and unfortuantely that's picked up by git, which means that when I try to work on the project in VSCode, it asks me to commit all of these cache files. clangd can be (and is) used by other IDEs as well, so it will help with those too; though I do not have experience with others.

[madd-games]

Please note that the Linux build in CI/CD is failing because it 404s on the GTK debian package at the moment, for some reason that's irrelevant to this PR.

[guillaumechereau]

Thanks!